Network health checker

ABSTRACT

A method of generating a network topology map in a datacenter comprising a network manager server and a set of host machines is provided. Each host machine hosts a set of data compute nodes (DCNs). The method receives information regarding the configuration of each of a set of logical networks from the network manager server. Each logical network is connected to several DCNs. The method identifies logical connections configured between the DCNs using the configuration of the overlay networks. The method generates a network topology map based on the identified logical configuration. The network topology identifies the DCNs that are connected to each overlay network. The method displays the network topology map on a graphical user interface.

CLAIM OF BENEFIT TO PRIOR APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication 62/415,467, filed Oct. 31, 2016. U.S. Provisional PatentApplication 62/415,467 is incorporated herein by reference.

BACKGROUND

With the advent of network virtualization and software-definednetworking (SDN), new issues are surfacing in the field of networktroubleshooting. The number of physical or virtual nodes and entitieshas increased by leaps and bounds in a software-defined datacenter(SDDC) network. Virtual switches, virtual routers, edge gateways,distributed firewalls, load-balancers, etc., are just some of the newnodes that add to the complexity of network troubleshooting. As aresult, network troubleshooting has become even more painful andcumbersome. Two of the major issues in SDDC network troubleshooting arelack of proper network topology diagrams and lack of a proactive networkhealth monitoring system.

Traditional networking provides a basic topology diagram to provide anidea of which virtual machine (VM) port or kernel port is located onwhich host. The basic topology may also indicate the virtualswitch/virtual distributed switches and physical network interface card(PNIC) that these ports are connected to. However, in a SDN there are alot more entities coming into the picture and understanding how theseentities are tied together is a key to network troubleshooting.

In a majority of SDDC network issues escalated by customers, there isneed for a proper topology diagram for efficient troubleshooting.Currently, there are no tools for generating the topology for SDDCnetworks. The collection of topology information from customers is donepurely manually, which opens up a lot of opportunities for introducingerrors.

There is a lot of back and forth with customers to get the customers'network topology information or diagrams. Even after putting in so muchtime and effort there is always some missing topology information.Sometimes, the information shared by the customer is incorrect. Evenafter collecting all the information from the customer, it is possiblethat the SDDC engineers may interpret the information incorrectly asthere is no standard way to represent the topology. Some customers don'teven have a topology diagram to share with the SDDC support personnel.The support personnel have to look at their environment and manuallybuild it on their own environment. The user may also misconfigure thetopology. It is possible that users might sometime connect VMs/logicalswitches/distributed logical routers incorrectly since they do not havea way to visualize the topology.

The errors introduced by manual efforts leads to missing pieces ofcritical topology data. This wastes a lot of engineering time andresources in the attempts to get proper topology information formcustomers. The errors introduced by manual efforts also lead tomisdiagnoses of network problems reported by the customers.

Another major issue in SDDC network troubleshooting is the lack ofproactive network health monitoring system. The support personnel cometo the realization that a network issue has occurred only after thedamage is done. There are no tools to proactively monitor the health ofthe network overlay links. For instance, parameters such as reachabilityof packets between any two VMs running on a network platform, latencyvariation of logical links between any two VMs, and maximum transmissionunit (MTU) variation of logical links between any two VMs cannot becurrently monitored.

BRIEF SUMMARY

Some embodiments provide an automated framework for dynamically learninga logical network topology and figuring out the configured logical linksin a datacenter. Two VMs have a configured logical link between themwhen the two VMs are connected either via a logical switch or adistributed logical router. A topology generator receives theinformation regarding the logical network configuration from network andcompute manger servers of the datacenter. The topology generatoridentifies the logical connections between different network entitiessuch as VMs, logical switches, logical routers, and edge gateways basedon the logical received network configuration.

The topology generator generates a network topology diagram for thelogical network by using the identified logical connections. Thetopology generator displays the topology diagram on a graphical userinterface and dynamically updates any changes to the logical networktopology. The logical network in some embodiments is an overlay network.

Some embodiments provide a health monitor on each host machine. Thehealth monitor receives the source and destination information for theconfigured logical links between each VM on the host and other VMs inthe datacenter. The health monitor proactively monitors the configuredlinks by periodically sending probe packets (e.g., spoof Internetcontrol message protocol (ICMP) packets) to each destination VM.

Based on the responses or lack of responses to the spoof ICMP packets,the health monitor deducts the reachability, latency, and MTUconfiguration issues on the logical links. The health monitor sends thestatus of each logical link to the topology generator to visuallyidentify the health status of the logical link on the topology diagram.

The preceding Summary is intended to serve as a brief introduction tosome embodiments of the invention. It is not meant to be an introductionor overview of all inventive subject matter disclosed in this document.The Detailed Description that follows and the Drawings that are referredto in the Detailed Description will further describe the embodimentsdescribed in the Summary as well as other embodiments. Accordingly, tounderstand all the embodiments described by this document, a full reviewof the Summary, Detailed Description and the Drawings is needed.Moreover, the claimed subject matters are not to be limited by theillustrative details in the Summary, Detailed Description and theDrawing.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features of the invention are set forth in the appendedclaims. However, for purposes of explanation, several embodiments of theinvention are set forth in the following figures.

FIG. 1 conceptually illustrates a network health checker in someembodiments.

FIG. 2 conceptually illustrates a topology generator in someembodiments.

FIG. 3 illustrates an example of an overlay topology diagram generatedby the topology generator in some embodiments.

FIG. 4 conceptually illustrates a graphical user interface (GUI) that isprovided by the topology generator in some embodiments.

FIG. 5 illustrates a dynamic change in the topology diagram of FIG. 4when a new database VM is added to the application tier.

FIG. 6 conceptually illustrates a process for generating a networktopology diagram in some embodiments.

FIG. 7 conceptually illustrates a process for sending logical linkinformation from the topology monitor to the health monitor andreceiving of link health status from the health monitor in someembodiments.

FIG. 8 conceptually illustrates a spoof ICMP packet that is sent fromthe health monitor on behalf of a particular VM to one of the VMs thatis connected to the particular VM in some embodiments.

FIG. 9 conceptually illustrates a process for generating spoof ICMPpackets and examining the corresponding responses.

FIG. 10 conceptually illustrates a process for generating spoof ICMPpackets and examining the corresponding responses for the determininglatency issues with the configured links.

FIG. 11 conceptually illustrates a process for generating spoof ICMPpackets and examining the corresponding responses for the determiningMTU issues with the configured links.

FIG. 12 conceptually illustrates a sequence flow diagram for a healthchecker of some embodiments that includes a topology generator and oneor more health monitors.

FIG. 13 conceptually illustrates an electronic system with which someembodiments of the invention are implemented.

DETAILED DESCRIPTION

In the following detailed description of the invention, numerousdetails, examples, and embodiments of the invention are set forth anddescribed. However, it will be clear and apparent to one skilled in theart that the invention is not limited to the embodiments set forth andthat the invention may be practiced without some of the specific detailsand examples discussed.

Some embodiments provide a topology generator that receives theinformation regarding the logical network configuration, identifies thelogical connections between different network entities such as VMs,logical switches, logical routers, and edge gateways, and generates anetwork topology diagram for the logical network by using the identifiedlogical connections.

Some embodiments provide a network health monitor that receives theinformation about the source and destination VMs on each configuredlogical link of a logical network. The health monitor proactivelymonitors the configured links by periodically sending probing packets(e.g., spoof ICMP packets). Based on the reply, the health monitordeducts the reachability, latency and MTU configuration issues on thelogical links.

FIG. 1 conceptually illustrates a network health checker in someembodiments. The figure also shows a cluster 150 of several hostmachines 160 in a datacenter. Each host machine 160 hosts one or moreVMs 165. As shown, the health checker has two components: a front-endtopology generator 105 and one or more health monitors 110. The topologygenerator 105 receives network configuration information from networkmanager 115 through a set of application programming interfaces (APIs).The topology generator also receives information about the VMs that arehosted on each host from the compute manager 120.

The topology generator identifies the logical connections between theVMs 165 based on the information received from the network manager andcompute manger. The topology generator generates the network topology170. For instance, the topology generator generates the network topologyas a report and/or displays the network topology as a connection diagramon a graphical user interface (e.g., at one or more administrator'sterminals).

Several more detailed embodiments of the invention are described below.Section I describes network topology generation. Section II describeslogical link health monitoring. Finally, Section III describes anelectronic system with which some embodiments of the invention areimplemented.

I. Network Topology Generation

In some embodiments, the topology generator 105 is implemented as anadapter to a cloud management tool that provides APIs to the compute andnetwork managers. FIG. 2 conceptually illustrates a topology generatorin some embodiments. The topology generator in the example of FIG. 2 isimplemented as an adapter of a cloud management tool 205. The cloudmanagement tool 205 provides a set of APIs to compute and networkmanagers in order to create and manage different compute and networkentities such as VMs, logical networks, managed forwarding elements,etc. In other embodiments, the topology generator is part of otherapplications available in the framework.

As shown, the cloud management tool utilizes a set of representationalstate transfer (REST or RESTful) application programming interfaces withthe network manager 115. The network manager in some embodiments is avirtualized server that is installed on one or more physical servers andis used to define and/or manage the networking resources of adatacenter. Each REST API identifies a resource or a service by a uniqueidentifier and indicates what actions have to be performed on theidentified resource or service. In some embodiments, the topologygenerator utilizes the APIs provided by the cloud management tool 205 toreceive the logical networks mappings 235 from the network manager 115.For instance, the logical network mappings may be provided by thenetwork manager by identifying the VMs, forwarding elements, logicalrouters, and gateways that are configured to be on the same logicalnetwork (or the same overlay network).

The cloud management tool 205 also utilizes a set of APIs to communicatewith the compute manager 120. The compute manager in some embodiments isa virtualized server that is installed on one or more physical servers.The compute manager is used to manage compute infrastructure (e.g., thehosts and VMs) of a datacenter. In some embodiments, the topologygenerator utilizes the APIs provided by the cloud management tool 205 toreceive the VM to host mappings 230 from the compute manager 120.

The topology generator gathers information regarding the VM to hostmappings and network configuration (for example the configuration ofoverlay networks) from the compute and network managers and generates atopology diagram for the network. In the following discussions, theoverlay networks are used as an example of a logical network. It shouldbe understood that the topology generator can gather informationregarding other types of logical networks.

An overlay network is a network virtualization technology that achievesmulti-tenancy in a computing environment. Examples of overlay networksinclude Virtual eXtensible LAN (VXLAN), Generic Network VirtualizationEncapsulation (GENEVE), and Network Virtualization using Generic RoutingEncapsulation (NVGRE). For instance, VXLAN is an L2 overlay scheme overa Layer 3 (L3) network. VXLAN encapsulates an Ethernet L2 frame in IP(MAC-in-UDP encapsulation) and allows VMs to be a part of virtualized L2subnets operating in separate physical L3 networks. Similarly, NVGREuses Generic Routing Encapsulation (GRE) to tunnel L2 packets over L3networks.

The followings are examples of API calls that the topology manager makesin some embodiments to gather information regarding the overlay networklinks:

https://<CmpMgr IP>/sdk (CmpMgr API call)

https://<NtwrkMgr IP>/api/4.0/edges (Rest API call)

where CmpMgr IP is the Internet Protocol (IP) address of the computemanager, NtwrkMgr IP is the IP address of the network manager, sdk is anexemplary software development kit that interfaces with the computemanager, and api/4.0/edges is an exemplary API used to interface withthe network manager. In operation, each API call has a set of parametersto inquire about different network and compute configurations of thedatacenter.

Using these API calls, the topology generator identifies differentmappings in the network deployment. The topology generator identifiesmappings such as VM to host, VM to forwarding element, forwardingelement to distributed logical router (DLR), and DLR to edge gateway.

The VM to host mappings determines the VMs 165 that are hosted on eachhost 160. The VM to forwarding element mappings, forwarding element toDLR mappings, and DLR to edge gateway mappings identify the logicallinks of the network. In some embodiments, each host machine in thenetwork includes virtualization software (e.g., a hypervisor) forvirtualizing physical resources of the host machine and a host managedforwarding element (MFE) for forwarding network traffic (e.g., datamessages) to and from the virtual machines. The host MFE, in someembodiments, operates within the virtualization software. In addition,some host machines include one or more VMs that connect to the host MFE.

In some embodiments, the host MFEs are configured to logically forwarddata messages according to logical forwarding elements (e.g., logicalswitches, logical routers) configured by an administrator of the networkto implement a logical forwarding element (LFE) of a logical network. Insome embodiments, the logical network is an abstract conception of anetwork generated by an administrator, and the logical network isimplemented in a virtualized, distributed manner in a managed physicalinfrastructure (e.g., in a multi-tenant datacenter). That is, the VMsthat connect to the LFEs may reside on various different host machineswithin the infrastructure, and MFEs (e.g., software virtual switches)operating on these host machines implement some or all of the logicalforwarding elements (logical switches, logical routers, etc.).

In the virtualization field, some refer to software switches as virtualswitches as these are software elements. However, in this specification,the software forwarding elements are referred to as MFEs, in order todistinguish them from LFEs, which are logical constructs that are nottied to the physical world. An MFE forwards packets in a physicalnetwork whether or not it is implemented in software while an LFEforwards packets in a logical network, which is logically decoupled orabstracted from the physical network. In other words, the softwareforwarding elements exist and operate in the physical world, whereas anLFE is a logical representation of a forwarding element that ispresented to a user when designing a logical network.

In some embodiments, several MFEs distributed throughout the networkimplement tenant's LFEs, where each MFE is a local instantiation of anLFE that operate across different host machines and can perform OpenSystems Interconnection (OSI) model Layer 2 (L2)/Layer 3 (L3) packetforwarding between VMs on the host machine or on different hostmachines. An LFE is sometimes referred to as a virtual distributedswitch (VDS).

A DLR provides a one-hop routing that enables a packet to be sentbetween two LFEs without going through a gateway. A DLR handles theeast-west traffic within a data center. East-west traffic is thecommunication between virtual machines within the datacenter.

An edge gateway is a virtual router that resides at the edge of thenetwork (e.g., as an interface to external networks) and providesnetwork services such as dynamic host configuration protocol (DHCP),firewall, network address translation (NAT), static routing, virtualprivate network (VPN), and load balancing.

Using the identified mappings (i.e., VM to host, VM to forwardingelement, forwarding element to DLR, and DLR to edge gateway mappings)the topology generator builds a parent-child relationship betweendifferent entities (such as VMs, MFEs, LFEs, DLRs, edge gateways, etc.)in the overlay network. This parent-child relationship is then used togenerate an overlay topology diagram that is presented (e.g., as aconnection diagram) in a graphical user interface. FIG. 3 illustrates anexample of an overlay topology diagram generated by the topologygenerator in some embodiments. The figure shows the network topology ofa 3-tiered enterprise application that includes a web tier, anapplication (app) tier, and a database (db) tier. The web tier includesthe presentation interface, the application tier includes the businesslogical, and the database tier includes the databases and manages readand write to these databases.

In the example of FIG. 3, the web tier includes two VMs 315-320 for anengineering department. The two VMs 315-320 are connected by theweb-tier LFE 345. The application tier includes three VMs 325-335 forthe engineering department. The three VMs 325-335 are connected by theapp-tier LFE 350. The database tier includes two VMs 305-310 for theengineering department. The two VMs 305-310 are connected by the db-tierLFE 340.

The three LFEs 340-350 are connected to each other through DLR 355. TheDLR is connected to an edge gateway 360. Each one of the connections inthe figure is identified by the topology generator through API calls tothe compute and network manager. The diagram of FIG. 3 in someembodiments is displayed on a graphical user interface (GUI).

FIG. 4 conceptually illustrates a GUI that is provided by the topologygenerator in some embodiments. As shown, the GUI includes a display area405 for displaying the topology diagram 410. The GUI also displays alist 415 of the objects identified by the topology generator. The listis displayed as a report that shows different properties of each objectand allows a user (e.g., an administrator) to apply different filters420, display and assign different policies 425 to the objects, etc.

As shown, the tab 430 labeled “sample storage topology graph” iscurrently selected in order to display the topology graph. The GUI alsoprovide other options to show data collection results 435, to show andchange user recommendation 440, and perform health monitoring 445.

The topology generator in some embodiments dynamically accounts for anychanges in the overlay network. The topology generator 105 (shown inFIGS. 1 and 2) periodically queries network manager 115 and computemanager 120 using their APIs and dynamically changes the topologydiagram if there are any changes in the topology.

FIG. 5 illustrates a dynamic change in the topology diagram of FIG. 4when a new database VM is added to the application tier. As shown, thetopology diagram is dynamically updated to show the newly addedRheld-DB3 VM 505.

FIG. 6 conceptually illustrates a process 600 for generating a networktopology diagram in some embodiments. The process in some embodiments isperformed by a topology generator such as the topology generator shownin FIGS. 1 and 2. As shown, the process receives (at 605) VM to hostmappings. For instance, the topology generator in FIG. 2 receives theidentification of the VMs that are hosted on each host using the computemanager APIs.

The process then receives (at 610) the logical networks configurations.For instance, the topology generator in FIG. 2 receives theconfiguration of each overlay network (or other types of logicalnetwork) from the network manager using the REST APIs. In someembodiments, each host machine may host VMs that belong to severaldifferent tenants. Each tenant typically generates one or more logicalnetworks for connecting the VMs of the tenant. Each of these logicalnetworks can be implemented by defining an overlay network. In someembodiments, the topology generator receives the information regardingthe VMs that are connected to each overlay network from the networkmanager through the REST API. In other embodiments, the topologygenerator receives the overlay networks configurations from the networkmanager (or other entities in the datacenter) through means other thanmaking REST API calls to the network manager.

Next, the process identifies (at 615) the logical connections betweendifferent entities of each logical network based on the logical networkconfiguration. For instance, the topology generator identifies groups ofVMs, LFEs, DLRs, and edge gateways that are on the same logical network(or are connected to the same overlay network) as logically connected toeach other.

The process then generates (at 620) the network topology for eachlogical network using the logical connections between different entitiesof the logical network such as VMs, LFEs, DLRs, and edge gateways. Forinstance, the topology generator generates a report (e.g., a list, atable, a diagram) for the logical network topology. Examples of topologydiagrams where discussed above by reference to FIGS. 3-5.

The process then determines (at 625) whether any changes in the VM tohost mappings or the logical network configuration has occurred. Forinstance, the process periodically performs operations similar tooperations 605-615 to determine whether there was any change in eachlogical network configuration. For instance, a new VM may be added to alogical network, an existing VM may removed or otherwise taken offline,a VM may migrate from one host to another host, there may be changes tothe configuration of LFEs, DLRs, and gateways in a logical network.

If not, the process proceeds back to 625 (e.g., after a wait period).Otherwise, the process updates (at 630) the topology diagram of eachlogical network based on the changes. For instance, the process maydetect that a new VM is added to a logical network and update thetopology map of the logical network as discussed above by reference toFIG. 5.

The topology generator in some embodiments finds out the configured (orlegitimate) links between the VMs. Two VMs are said to have a configured(or legitimate) logical link between them if they are connected throughthe LFEs and DLRs in the same logical network (i.e., the logical networkthe is connected to external networks by an edge gateway. In someembodiments, the topology generator also consider other factors such asfirewall rules in other to further refine the configured logical linksas the logical links between the VMs that can communicate with eachother.

The topology generator determines which VM can talk to which other VMsbased on the topology information gathered by the topology generator.For every VM in the topology, the topology generator determines allother VMs that the VM is connected to and marks the links between theVMs as configured (or legitimate logical links). The topology generatorcan trigger a health monitor in each host and pass the informationregarding the configured logical links to the health monitor.

II. Logical Link Health Monitoring

In some embodiments, the health monitor in each host (e.g., healthmonitors 110 on hosts 160 shown in FIGS. 1-2) is implemented as a kernelmodule. In some embodiments, the health monitor in each host sends probepackets on the identified configured (or legitimate) logical links todetect network issues like link failure, MTU mismatch and unacceptablelatencies. In some embodiments, probe packets are spoof ICMP packets.ICMP is a protocol that is used by network devices to send errormessages to indicate that a destination cannot be reached or a requestedservice is not available. It should be understood that other types ofprobe packets can be sent instead of spoof ICMP packets.

The health monitor then report the success or failures back to thetopology generator. Based on the reported results, the topologygenerator changes the colors of the logical links. For instance, a linkcolor of green shown on a topology diagram (e.g., topology diagramsshown in FIGS. 3-5) may indicate that the logical link health is good.Another color (e.g., yellow) may indicate that the logical link haslatency issues. Another color (e.g., orange) may indicate that thelogical link has MTU issues. Another color (e.g., red) may indicate thatthe logical link is down.

The generated topology diagram is also useful for administrators tovisualize any topology related misconfigurations. If the generatedtopology is not what the administrator expected, then the administratorcan figure out if the VMs are connected to incorrect LFEs or if the LFEsare connected to wrong DLRs.

As discussed above by reference to FIG. 1, each host 160 has a logicallink health monitor 100. The health monitors in some embodiments areimplemented as kernel modules. Each health monitor injects ICMP spoofpackets on all the configured (or legitimate) logical links. The healthmonitor monitors the health of these logical links by sending the ICMPspoof packets at regular intervals. For instance, the logical linkhealth monitor interval in some embodiments may be similar to theinterval of other health monitors (e.g., a health monitor used forLFEs). In some embodiments, the interval at which the health monitorinjects the spoof packets is configurable.

In some embodiments, the topology generator retrieves additionalinformation such as media access cone (MAC) and IP address of each VMfrom the compute manager (e.g., by using the compute manager APIs). Oncethe topology generator identifies all configured logical links, thetopology generator sends the configured logical link information foreach VM that resides on a host to the health monitor of the host.

For instance, if a VM is connected to n other VMs in the topology (i.e.,the VM has n configured logical link and can send packets to the n VMs),the topology generator pushes n pairs of (source VM, destination VM)details to the health monitor of the host where the VM resides. For eachlink, the topology generator sends source MAC address, destination MACaddress, source IP address, and destination IP address of the link tothe health monitor. The source VM is the VM that resides on the samehost as the health monitor. If both VMs on a configured logical link areon the same host, the topology generator in some embodiments sends twosets of information, each set of information considers one of the VMs asthe source VM and the other VM as the destination VM for the purpose ofsending the ICMP spoof packets.

Different embodiments utilize different methods to pass the configuredlogical link information from the topology generator to the healthmonitors. Some embodiments use a daemon that runs on each host, receivesthe configured logical link information from the topology generator, andpasses the information to the health monitor. Other embodiments utilizea kernel shell utility (that is used to access different data structuresand variables) in order to pass the information from the topologygenerator to the health monitor.

FIG. 7 conceptually illustrates a process 700 for sending logical linkinformation from the topology monitor to the health monitor andreceiving of link health status from the health monitor in someembodiments. The process in some embodiments is performed by a topologygenerator such as the topology generator shown in FIGS. 1 and 2. Asshown, the process receives (at 705) configuration information for eachconfigured logical link between a source VM on the host and any otherVMs from the compute manager. For instance, the process receives thesource MAC address, destination MAC address, source IP address, anddestination IP address for each link from the compute manager 120 usingcompute manager APIs as discussed above by reference to FIG. 2.

The process then sends (at 710) the information for each configuredlogical link for each source VM to the health monitor on the host of theVM. The process then receives (at 715) link health status from thehealth monitor. The process then updates (at 720) the topology map basedon the received link health status. For instance, the process sets thecolor of each logical link in order to visually identify the link ashealthy, as having latency issues, as having MTU issues, or as failed.The process then proceeds to 705, which was described above.

A. Sending Spoof ICMP Packets to Monitor the Network Health

Once the health monitor in a host receives the information for theconfigured logical links for each host, the health monitor generatesspoof ICMP packets and sends the packets to other VMs (that may or maynot be of the same host) over the configured logical links. ICMP packetsare typically encapsulated in a Layer 3 packet (or “network” layerpacket such as IP version 4 (IPv4) or IP version 6 (IPv6) packet), whichis in turn encapsulated in a Layer 2 packet (or data link layer packetsuch as Ethernet packet). When Overlay networks are used, the ICMPpackets are encapsulated in a Layer 3 packet, which is encapsulated in aLayer 2 packet, which is in turn encapsulated in an overlay network(e.g., VXLAN or GENEVE) packet.

FIG. 8 conceptually illustrates a spoof ICMP packet that is sent fromthe health monitor on behalf of a particular VM to one of the VMs thatis connected to the particular VM in some embodiments. A similar packetis periodically sent to every VM that is connected to the particular VMthrough a configured logical link.

As shown, the spoof ICMP packet 855 is encapsulated in a Layer 3 (ornetwork layer) packet 845, which is encapsulated in a Layer 2 (or datalink layer) packet 840, which is in turn encapsulated in an overlaynetwork packet 850. Layer 2 encapsulation 840 includes source MACaddress 805 and destination MAC address 810. Layer 3 encapsulation 845includes source IP address 815 and destination IP address 820. Thesource MAC address 805 and the source IP address 815 are the MAC and IPaddresses of the VM on whose behalf the health monitor is sending thepacket. The destination MAC address 810 and the destination IP address820 are the MAC and IP addresses of the destination VM (i.e., the VM onthe other end of the configured logical link).

The spoof ICMP packet 855 includes a header 825 and a payload 860. Thespoof ICMP packet includes an identifier 830, which is an identifier (ora magic number) that is added to the spoof ICMP packets in order todistinguish the spoof packets that the health monitor injects with theregular ICMP packets. In some embodiments, the spoof packet identifieris the same for all spoof packets that are injected by the healthmonitors.

In some embodiments, the identifier of the spoof packets 825 is includedin the first few bytes of the spoof ICMP packet payload 860. The healthmonitor also embeds a unique sequence number 835 in the spoof ICMPpacket (e.g., in the sequence field of the ICMP packet) in order touniquely identify various packets the health monitor injects. Once thehealth monitor builds the packet, the health monitor injects this packetthrough the MFE port that the source VM is connected to. As discussedabove, each host machine in the network includes an MFE for forwardingnetwork traffic to and from the virtual machines. Each VM in the hostmachine connects to a port of the host MFE through a virtual networkinterface card (VNIC). The host MFEs are then configured to logicallyforward data messages according to LFEs configured by an administratorof the network in order to implement the LFEs. The health monitorgenerates the spoof ICMP packets for each VM and injects the packetsthrough the MFE port used by the source VM in order for the packet as ifthe packet was initiated by the source VM.

B. Examining Responses to the Spoof ICMP Packets

In order to examine the responses receive for the spoof packet, someembodiments use a kernel-level packet handling process (referred to asan IOChain) on the MFE port to which each source VM is connected. TheIOChain acts as a filter and processes the responses to spoof ICMPpackets that were injected into the MFE port. The IOChain only examinesthe ICMP packets that have the identifier (or magic number) 830 shown inFIG. 8 associated with the spoof ICMP packets in their payload 860.

The health monitor also maintains a state for all packets injected andthe time at which the packets are sent (e.g., in the form of a pendinglist) in order to compare the packets with their correspondingresponses. The health monitor also has a timer, which expires at regularinterval in order to check the pending list to determine which packetshave got responses and which packets didn't.

If the response to a packet is received after a first threshold (e.g.,200 millisecond, 400 millisecond, etc.), the link between source anddestination VMs is marked as having latency issues. If there is noresponse after a second threshold (e.g., 800 millisecond, 1 second,etc.) then the link is marked as down.

In order to determine MTU issues, the health monitor in some embodimentssends several packets (e.g., 3 packets) with varied sizes to eachdestination VM. The MTU is the maximum number of bytes an individualpacket (or datagram) can have on a communication link. The healthmonitor, for example, sends packets with 1500, 1600, and 9000 bytes perconfigured logical link. If none of these packets receives an MTUfailure response (e.g., an ICMP packet indicating the datagram is toobig), the link is marked as free of MTU issues.

Once the health monitor analyses the responses (or lack thereof) to thespoof ICM packets, the health monitor send the results of the analysisback to the topology generator so that the topology generator can alertthe user (e.g., by changing the color of the logical link on thetopology diagram.

FIG. 9 conceptually illustrates a process 900 for generating spoof ICMPpackets and examining the corresponding responses. The process in someembodiments is performed by a logical link health monitor such as any oflogical link health monitors 110 in FIGS. 1-2. As shown, the processreceives (at 905) information for each configured logical link between aVM (referred to as source VM) on the host and any other VMs on the sameor other hosts. For instance, the process receives the source MACaddress, destination MAC address, source IP address, and destination IPaddress for each link from the topology generator.

The process then generates (at 910) a spoof ICMP packet to check thelatency of each configured logical link between a source VM on the hostand any other VMs on the same or other hosts. The process then generates(at 915) one or more spoof ICMP packet to check the MTU issues of eachconfigured logical link between a source VM on the host and any otherVM.

The process then waits (at 920) for a predetermined amount of time. Forinstance, the process set a timer and wakes up (or gets triggered) atthe expiration of the timer. The process then examines (at 925) allreceived responses for the spoof ICMP packets. Details of theexamination of the packets sent for latency check and MTU issues checkare described below by reference to FIGS. 10-11, respectively.

The process then sends (at 930) the link health status to the topologygenerator. The process then receives (at 935) updated information (ifany) for each configured logical link between a source VM on the hostand any other VMs. For instance, as described above, the topologygenerator periodically receives updated configuration information fromthe compute and network managers and pushes the updated information tothe health monitor. The process then proceeds to 1008, which wasdescribed above.

FIG. 10 conceptually illustrates a process 1000 for generating spoofICMP packets and examining the corresponding responses for thedetermining latency issues with the configured links. The process insome embodiments is performed by a logical link health monitor such asany of logical link health monitors 110 in FIGS. 1-2.

As shown, the process generates (at 1005) a spoof ICMP packet to checkthe latency of each configured logical link between a source VM on thehost and any other VM. The process then starts (at 1010) a latency timerand a failure timer for each spoof ICMP packet and injects the packetinto the MFE port connected to the source VM.

The process then waits (at 1015) for a predetermined amount of time. Theprocess then examines (at 1020) all received responses for the spoofICMP packets. The process marks (at 1025) the logical links for whichthe response is received before the expiration of the correspondinglatency timer as having acceptable latency.

The process marks (at 1030) the logical links for which the response isreceived after the expiration of the corresponding latency timer andbefore the expiration of the failure timer as having latency issues. Theprocess marks (at 1035) the logical links for which no response isreceived before the expiration of the failure timer as being failed. Theprocess then ends. Process 1000 is periodically performed by the healthmonitor in order to dynamically monitor the health of the logical links.

FIG. 11 conceptually illustrates a process 1100 for generating spoofICMP packets and examining the corresponding responses for thedetermining MTU issues with the configured links. The process in someembodiments is performed by a logical link health monitor such as any oflogical link health monitors 110 in FIGS. 1-2.

As shown, the process generates (at 1105) several spoof ICMP packetswith different sizes to check the MTU issues of each configured logicallink between a source VM on the host and any other VM. The process theninjects (at 1110) the packets into the MFE port connected to the sourceVM.

The process then waits (at 1115) for a predetermined amount of time. Theprocess then examines (at 1120) all received responses for the spoofICMP packets. The process marks (at 1125) the logical links for which noresponse indicating the packet is too big is received as links having noMTU issues. The process marks (at 1130) the logical links for which aresponse to a packet with a particular size is received indicating thepacket is too big as a link that has MTU issues with packets of the sameor larger size than the particular size. The process then ends. Process1100 is periodically performed by the health monitor in order todynamically monitor the health of the logical links.

FIG. 12 conceptually illustrates a sequence flow diagram for a healthchecker of some embodiments that includes a topology generator and oneor more health monitors. The figure shows a network manager 1205, acompute manager 1210, a topology generator 1215, and two health monitors1220 and 1225. Health monitor 1220 runs on a host that is hosting VM1and VM2. Health monitor 1225 runs on a host that is hosting VM3 and VM4.

As shown, topology generator 1215 at regular intervals (as shown by1232) sends (at 1230 and 1235) API requests to the network manager 1205and compute manager 1210. Topology generator 1215 parses (at 1240) theAPI data to generate the overlay topology.

Topology generator 1215 also parses (at 1245) the APIs data to determinethe configured logical links. Topology generator 1215 sends (at 1250)information for all configured logical links for VM3 and VM4 to healthmonitor 1225. Topology generator 1215 also sends (at 1255) informationfor all configured logical links for VM1 and VM2 to health monitor 1220.

Health monitor 1225 sends (at 1260) probe packets at regular intervalsto determine the health of the configured logical links of VM3 and VM4.Health monitor 1220 sends (at 1265) probe packets at regular intervalsto determine the health of the configured logical links of VM1 and VM2.

Health monitor 1220 sends (at 1270) the status of the logical links'health to the topology generator 1215. Health monitor 1225 sends (at1275) the status of the logical links' health to the topology generator1215. The topology monitor updates (at 1280) the topology based on thelink statuses and generate alerts if needed.

C. Advantages of Using Dynamic Topology Generator and Logical LinksHealth Monitor

The use of a health checker that includes the dynamic topology generatorand logical link health monitoring reduces the time-to-relief with someof the most common network escalations dealing with latencies, MTUconfiguration issues, and link down issues. The administrators areenabled to monitor the health of the logical links and troubleshoot theissues.

Some embodiments also trigger log collection from the destination hostsof the logical links that have issues and use the logs fortroubleshooting. The topology generator generates overlay networktopology diagrams, which reduces the back and forth questions with thecustomers in order to understand the customers' network deployment. Thistopology diagram generated by the topology generator enables engineers,customer support services, as well as the customers to get a completepicture of the network overlay topology. The generated topology diagramis also useful for administrators to check if there are any topologyrelated misconfigurations. All of these advantages in turn help avoidingany misdiagnoses of network issues.

The health checker is a proactive rather than demand-based way tomonitor the health of the overlay links. Existing solutions are ondemand based. The health checker proactively and periodically measuresthe health of the overlay links and notifies the administrators of thefailed links or links with latency issues. Since the logical link healthmonitor is aware of the topology, the reachability test is performedwith optimized number of test packets.

Instead of troubleshooting networking issues after the networking issuebecomes visible, the disclosed health checker performs constantmonitoring and notifies the administrators of the failed logical links,close to the failure time. The disclosed health checker pinpoints andidentifies the exact link failure.

One of the advantages of the disclosed health checker is the informationrequired to generate the topology is gathered using the REST APIs andcompute manager APIs as discussed above by reference to FIG. 2. There isno need to send probe packets to figure out the topology and noadditional overhead is generated by sending probe determine thetopology. In addition, sending probe packets only finds out the topologybetween two IP endpoints. In contrast, the disclosed health checkerfigures out the complete topology by analyzing the information receivedthrough the APIs from the compute and network manager servers.

The topology diagrams discussed above by reference to FIGS. 3-5 presentsa simple way to the user to visualize all entities (VMs, LFEs, DLRs,edge gateways) in each overlay network and how the entities areconnected to each other. There is no need to specify which two endpointsthe user wants to monitor. The health checker monitors the whole networkand alerts the user when there is a health issue.

The health checker does not require any changes to the overlay networkor tunneling protocols in order to monitor the health of the overlaylinks. The health checker uses the available configuration informationgathered from the compute and network manager servers.

III. Electronic System

Many of the above-described features and applications are implemented assoftware processes that are specified as a set of instructions recordedon a computer readable storage medium (also referred to as computerreadable medium). When these instructions are executed by one or moreprocessing unit(s) (e.g., one or more processors, cores of processors,or other processing units), they cause the processing unit(s) to performthe actions indicated in the instructions. Examples of computer readablemedia include, but are not limited to, CD-ROMs, flash drives, RAM chips,hard drives, EPROMs, etc. The computer readable media does not includecarrier waves and electronic signals passing wirelessly or over wiredconnections.

In this specification, the term “software” is meant to include firmwareresiding in read-only memory or applications stored in magnetic storage,which can be read into memory for processing by a processor. Also, insome embodiments, multiple software inventions can be implemented assub-parts of a larger program while remaining distinct softwareinventions. In some embodiments, multiple software inventions can alsobe implemented as separate programs. Finally, any combination ofseparate programs that together implement a software invention describedhere is within the scope of the invention. In some embodiments, thesoftware programs, when installed to operate on one or more electronicsystems, define one or more specific machine implementations thatexecute and perform the operations of the software programs.

FIG. 13 conceptually illustrates an electronic system 1300 with whichsome embodiments of the invention are implemented. The electronic system1300 can be used to execute any of the control, virtualization, oroperating system applications described above. The electronic system1300 may be a computer (e.g., a desktop computer, personal computer,tablet computer, server computer, mainframe, a blade computer etc.),phone, PDA, or any other sort of electronic device. Such an electronicsystem includes various types of computer readable media and interfacesfor various other types of computer readable media. Electronic system1300 includes a bus 1305, processing unit(s) 1310, a system memory 1320,a read-only memory (ROM) 1330, a permanent storage device 1335, inputdevices 1340, and output devices 1345.

The bus 1305 collectively represents all system, peripheral, and chipsetbuses that communicatively connect the numerous internal devices of theelectronic system 1300. For instance, the bus 1305 communicativelyconnects the processing unit(s) 1310 with the read-only memory 1330, thesystem memory 1320, and the permanent storage device 1335.

From these various memory units, the processing unit(s) 1310 retrieveinstructions to execute and data to process in order to execute theprocesses of the invention. The processing unit(s) may be a singleprocessor or a multi-core processor in different embodiments.

The read-only-memory 1330 stores static data and instructions that areneeded by the processing unit(s) 1310 and other modules of theelectronic system. The permanent storage device 1335, on the other hand,is a read-and-write memory device. This device is a non-volatile memoryunit that stores instructions and data even when the electronic system1300 is off. Some embodiments of the invention use a mass-storage device(such as a magnetic or optical disk and its corresponding disk drive) asthe permanent storage device 1335.

Other embodiments use a removable storage device (such as a floppy disk,flash drive, etc.) as the permanent storage device. Like the permanentstorage device 1335, the system memory 1320 is a read-and-write memorydevice. However, unlike storage device 1335, the system memory is avolatile read-and-write memory, such as random access memory. The systemmemory stores some of the instructions and data that the processor needsat runtime. In some embodiments, the invention's processes are stored inthe system memory 1320, the permanent storage device 1335, and/or theread-only memory 1330. From these various memory units, the processingunit(s) 1310 retrieve instructions to execute and data to process inorder to execute the processes of some embodiments.

The bus 1305 also connects to the input and output devices 1340 and1345. The input devices enable the user to communicate information andselect commands to the electronic system. The input devices 1340 includealphanumeric keyboards and pointing devices (also called “cursor controldevices”). The output devices 1345 display images generated by theelectronic system. The output devices include printers and displaydevices, such as cathode ray tubes (CRT) or liquid crystal displays(LCD). Some embodiments include devices such as a touchscreen thatfunction as both input and output devices.

Finally, as shown in FIG. 13, bus 1305 also couples electronic system1300 to a network 1325 through a network adapter (not shown). In thismanner, the computer can be a part of a network of computers (such as alocal area network (“LAN”), a wide area network (“WAN”), or an Intranet,or a network of networks, such as the Internet. Any or all components ofelectronic system 1300 may be used in conjunction with the invention.

Some embodiments include electronic components, such as microprocessors,storage and memory that store computer program instructions in amachine-readable or computer-readable medium (alternatively referred toas computer-readable storage media, machine-readable media, ormachine-readable storage media). Some examples of such computer-readablemedia include RAM, ROM, read-only compact discs (CD-ROM), recordablecompact discs (CD-R), rewritable compact discs (CD-RW), read-onlydigital versatile discs (e.g., DVD-ROM, dual-layer DVD-ROM), a varietyof recordable/rewritable DVDs (e.g., DVD-RAM, DVD-RW, DVD+RW, etc.),flash memory (e.g., SD cards, mini-SD cards, micro-SD cards, etc.),magnetic and/or solid state hard drives, read-only and recordableBlu-Ray® discs, ultra density optical discs, any other optical ormagnetic media, and floppy disks. The computer-readable media may storea computer program that is executable by at least one processing unitand includes sets of instructions for performing various operations.Examples of computer programs or computer code include machine code,such as is produced by a compiler, and files including higher-level codethat are executed by a computer, an electronic component, or amicroprocessor using an interpreter.

While the above discussion primarily refers to microprocessor ormulti-core processors that execute software, some embodiments areperformed by one or more integrated circuits, such as applicationspecific integrated circuits (ASICs) or field programmable gate arrays(FPGAs). In some embodiments, such integrated circuits executeinstructions that are stored on the circuit itself.

As used in this specification, the terms “computer”, “server”,“processor”, and “memory” all refer to electronic or other technologicaldevices. These terms exclude people or groups of people. For thepurposes of the specification, the terms display or displaying meansdisplaying on an electronic device. As used in this specification, theterms “computer readable medium,” “computer readable media,” and“machine readable medium” are entirely restricted to tangible, physicalobjects that store information in a form that is readable by a computer.These terms exclude any wireless signals, wired download signals, andany other ephemeral or transitory signals.

While the invention has been described with reference to numerousspecific details, one of ordinary skill in the art will recognize thatthe invention can be embodied in other specific forms without departingfrom the spirit of the invention. In addition, a number of the figures(including FIGS. 6-7 and 9-12) conceptually illustrate processes. Thespecific operations of these processes may not be performed in the exactorder shown and described. The specific operations may not be performedin one continuous series of operations, and different specificoperations may be performed in different embodiments. Furthermore, theprocess could be implemented using several sub-processes, or as part ofa larger macro process.

This specification refers throughout to computational and networkenvironments that include virtual machines (VMs). However, virtualmachines are merely one example of data compute nodes (DCNs) or datacompute end nodes, also referred to as addressable nodes. DCNs mayinclude non-virtualized physical hosts, virtual machines, containersthat run on top of a host operating system without the need for ahypervisor or separate operating system, and hypervisor kernel networkinterface modules.

VMs, in some embodiments, operate with their own guest operating systemson a host using resources of the host virtualized by virtualizationsoftware (e.g., a hypervisor, virtual machine monitor, etc.). The tenant(i.e., the owner of the VM) can choose which applications to operate ontop of the guest operating system. Some containers, on the other hand,are constructs that run on top of a host operating system without theneed for a hypervisor or separate guest operating system. In someembodiments, the host operating system uses name spaces to isolate thecontainers from each other and therefore provides operating-system levelsegregation of the different groups of applications that operate withindifferent containers. This segregation is akin to the VM segregationthat is offered in hypervisor-virtualized environments that virtualizesystem hardware, and thus can be viewed as a form of virtualization thatisolates different groups of applications that operate in differentcontainers. Such containers are more lightweight than VMs.

Hypervisor kernel network interface module, in some embodiments, is anon-VM DCN that includes a network stack with a hypervisor kernelnetwork interface and receive/transmit threads. One example of ahypervisor kernel network interface module is the vmknic module that ispart of the ESXi™ hypervisor of VMware, Inc.

One of ordinary skill in the art will recognize that while thespecification refers to VMs, the examples given could be any type ofDCNs, including physical hosts, VMs, non-VM containers, and hypervisorkernel network interface modules. In fact, the example networks couldinclude combinations of different types of DCNs in some embodiments.

In view of the foregoing, one of ordinary skill in the art wouldunderstand that the invention is not to be limited by the foregoingillustrative details, but rather is to be defined by the appendedclaims.

What is claimed is:
 1. A method of generating a network topology diagramin a datacenter comprising a network manager server and a set of hostcomputers, a set of data compute nodes (DCNs) executing on each hostcomputer, each of at least a plurality of the host computers executing aplurality of DCNs, the method comprising: receiving informationregarding a configuration of each of a set of logical overlay networksfrom the network manager server, each logical overlay network connectedto a plurality of DCNs; identifying logical connections configuredbetween the DCNs using the configuration of the logical overlaynetworks; generating the network topology diagram based on theidentified logical connections, the network topology diagram identifyingthe DCNs that are connected to each logical overlay network and linksconnected to the DCNs to represent the logical connections associatedwith the DCNs; sending health-check messages to each of the DCNs andanalyzing responses from the DCNs to the health-check messages toidentify the health of the links connected to the DCNs; and displayingthe network topology diagram on a graphical user interface, saiddisplaying comprising displaying at least one particular link connectedto one DCN differently than a plurality of other links connected toother DCNs in order to identify the health of the particular link. 2.The method of claim 1, wherein the datacenter further comprises acompute manager, the method further comprising: receiving informationregarding the DCNs that execute on each host computer from the computemanager server; wherein generating the network topology diagram furthercomprises using the information regarding the DCNs that execute on eachhost computer.
 3. The method of claim 2 further comprising: receivinginformation regarding changes to the configuration of each logicaloverlay network from the compute manager; updating the network topologydiagram based on the changes, and displaying the updated networktopology diagram on the graphical user interface.
 4. The method of claim2 further comprising: receiving information regarding a change in theDCNs that execute on a host computer from the compute manager server;updating the network topology diagram based on the changes, anddisplaying the updated network topology diagram on the graphical userinterface.
 5. The method of claim 1, wherein receiving informationregarding the configuration of each logical overlay network comprisesreceiving an identification of each DCN, each logical forwardingelement, each virtual logical router, and each edge gateway connected tothe logical overlay network.
 6. The method of claim 1, wherein eachlogical overlay network is an overlay network configured by a tenant ofthe datacenter.
 7. The method of claim 1, wherein sending thehealth-check messages comprises: sending spoof Internet control messageprotocol (ICMP) packets to each destination DCN; and analyzing responsesand lack of responses for each spoof ICMP packet to determine the healthof each configured logical link.
 8. The method of claim 7 furthercomprising: utilizing the determined health of each configured logicallink to visually identify a health status of each configured logicallink as one of healthy, having latency issues, having maximumtransmission unit (MTU) issues, and failed.
 9. The method of claim 7,wherein the spoof ICMP packets comprise spoof ICMP packets sent todetermine latency issues of each configured logical link, the methodfurther comprising: starting a first timer and a second timer at a timeof sending each spoof ICMP packet, the second timer having a longerexpiration time than the first timer; marking each logical link forwhich a response to the spoof ICM packet is received before anexpiration of the first timer as a health logical link; marking eachlogical link for which a response to the spoof ICM packet is receivedafter the expiration of the first timer and before an expiration of thesecond timer as a link having latency issues; and marking each logicallink for which no response is received to the spoof ICM packet beforethe expiration of the second timer as a failed logical link.
 10. Themethod of claim 7, wherein the spoof ICMP packets comprise spoof ICMPpackets sent to determine maximum transmission unit (MTU) issues, themethod further comprising: sending a plurality of spoof ICMP packetswith different sizes to each destination VM over each configured logicallink; marking each logical link as having an acceptable MTU when noresponse indicating a spoof ICMP packet is too big over the logicallink; and marking each logical link as having an MTU issue with aparticular packet size when a response is received to a spoof ICMPpacket with the particular size indicating the packet is too big.
 11. Anon-transitory machine readable medium storing a program that whenexecuted by at least one processing unit generates a network topologydiagram in a datacenter comprising a network manager server and a set ofhost computers, a set of data compute nodes (DCNs) executing on eachhost computer, each of at least a plurality of the host computersexecuting a plurality of DCNs, the program comprising sets ofinstructions for: receiving information regarding a configuration ofeach of a set of logical overlay networks from the network managerserver, each logical overlay network connected to a plurality of DCNs;identifying logical connections configured between the DCNs using theconfiguration of the logical overlay networks; generating the networktopology diagram based on the identified logical connections, thenetwork topology diagram identifying the DCNs that are connected to eachlogical overlay network; sending health-check messages to each of theDCNs and analyzing responses from the DCNs to the health-check messagesto identify the health of the links connected to the DCNs; anddisplaying the network topology diagram on a graphical user interface.12. The non-transitory machine readable medium of claim 11, wherein thedatacenter further comprises a compute manager, the program furthercomprising a set of instructions for: receiving an identification of theDCNs executed by each host computer from the compute manager server;wherein the set of instructions for generating the network topologydiagram further comprises a set of instructions for using theidentification of the DCNs to determine the DCNs that execute on eachhost computer.
 13. The non-transitory machine readable medium of claim12, the program further comprising sets of instructions for: receivinginformation regarding changes to the configuration of each logicaloverlay network from the compute manager; updating the network topologydiagram based on the changes, and displaying the updated networktopology diagram on the graphical user interface.
 14. The non-transitorymachine readable medium of claim 12, the program further comprising setsof instructions for: receiving information regarding a change in theDCNs that execute on a host computer from the compute manager server;updating the network topology diagram based on the changes, anddisplaying the updated network topology diagram on the graphical userinterface.
 15. The non-transitory machine readable medium of claim 11,wherein receiving information regarding the configuration of eachlogical overlay network comprises receiving an identification of eachDCN, each logical forwarding element, each virtual logical router, andeach edge gateway connected to the logical overlay network.
 16. Thenon-transitory machine readable medium of claim 11, wherein each logicaloverlay network is an overlay network configured by a tenant of thedatacenter.
 17. The non-transitory machine readable medium of claim 11,wherein the set of instructions for sending the health-check messagescomprises sets of instructions for: sending spoof Internet controlmessage protocol (ICMP) packets to each destination DCN; and analyzingresponses and lack of responses for each spoof ICMP packet to determinethe health of each configured logical link.
 18. The non-transitorymachine readable medium of claim 17, the program further comprising setsof instructions for: utilizing the determined health of each configuredlogical link to visually identify a health status of each configuredlogical link as one of healthy, having latency issues, having maximumtransmission unit (MTU) issues, and failed.
 19. The non-transitorymachine readable medium of claim 17, wherein the spoof ICMP packetscomprise spoof ICMP packets sent to determine latency issues of eachconfigured logical link, the program further comprising sets ofinstructions for: starting a first timer and a second timer at a time ofsending each spoof ICMP packet, the second timer having a longerexpiration time than the first timer; marking each logical link forwhich a response to the spoof ICM packet is received before anexpiration of the first timer as a health logical link; marking eachlogical link for which a response to the spoof ICM packet is receivedafter the expiration of the first timer and before an expiration of thesecond timer as a link having latency issues; and marking each logicallink for which no response is received to the spoof ICM packet beforethe expiration of the second timer as a failed logical link.
 20. Thenon-transitory machine readable medium of claim 17, wherein the spoofICMP packets comprise spoof ICMP packets sent to determine maximumtransmission unit (MTU) issues, the program further comprising sets ofinstructions for: sending a plurality of spoof ICMP packets withdifferent sizes to each destination VM over each configured logicallink; marking each logical link as having an acceptable MTU when noresponse indicating a spoof ICMP packet is too big over the logicallink; and marking each logical link as having an MTU issue with aparticular packet size when a response is received to a spoof ICMPpacket with the particular size indicating the packet is too big.